I did this for a math project in school about a month ago. Going with a 15 mile an hour wind we calculated that my LT-40 was doing about 83 km/h with a .46 FX and 10x6 prop on it. Against the wind we were going about 53 Km/h. So the average speed or the normal speed (without wind) would be about 68 Km/h just as kingwoodbarney said.